1. Field
The present invention relates to a solid oxide electrochemical cell such as, e.g., a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC), to a hydrogen electrode material for use in the cell, and to processes for producing the cell.
2. Description of Related Art
Examples of solid oxide electrochemical cells include solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). The solid oxide electrochemical cells have a high operating temperature (700-1,000° C.). Because of this, SOFCs are hopeful as a next-generation clean power generation system having a high power generation efficiency and reduced in CO2 generation. On the other hand, SOECs are hopeful in a high-efficiency hydrogen production method capable of yielding high-purity hydrogen in one step.
For producing a hydrogen electrode material for solid oxide electrochemical cells, a method is generally employed in which ceramic particles having ionic conductivity are mixed with metal particles having electronic conductivity. Other techniques have been disclosed which include one in which SDC (CeO2 doped with Sm2O3) particles having electron/oxide ion mixed conductivity are used and fine particles of nickel are highly dispersedly deposited on the surface of the SDC particles for the purpose of attaining higher performances (see J. Electrochem. Soc., 141, [2], 342-346, 1994).
In the technique in which the SDC material having mixed conductivity is used, nickel particles are formed in a porous material constituted of an SDC network by the impregnation method using, e.g., an aqueous metal salt solution. This technique has succeeded in reducing the size of nickel particles by at least one order of magnitude and in obtaining high catalytic activity with a smaller nickel addition amount and forming a complete electron network within the electrode. In addition, since SDC further has electronic conductivity, the boundary between each of all the fine nickel particles and the SDC theoretically functions as a three-phase boundary. There is a description in that document to the effect that bonding between the nickel and the SDC is relatively satisfactory. However, because nickel particles are formed by impregnation with the solution, burning, and reduction, the particles change in size with time or particle sintering occurs during the burning step, resulting in an uneven structure.
For reducing overvoltage and increasing catalytic activity in a hydrogen electrode, it is necessary to use finer metal particles (e.g., nickel particles) as a catalyst and thereby increase the number of active sites. However, in a high-temperature reducing atmosphere, the metal particles readily move, grow, and aggregate. Furthermore, it is difficult to incorporate nickel particles in an unnecessarily large amount partly because of a difference in the coefficient of thermal expansion. In addition, in case where abrupt oxidation has occurred, the formation of an oxide results in volume expansion and this may cause cell breakage.
The present inventors previously proposed a process for catalyst production based on reduction deposition from an Ni—Al composite oxide solid solution as a technique for reducing the size of nickel particles to improve catalytic activity, inhibiting catalyst sintering, and constituting an electrode in which components have the same coefficient of thermal expansion. However, the fine metal particles deposited are not in an interconnected state, and aluminum oxide, which is an insulator, is formed after the reduction deposition. Consequently, it has been necessary to cause conductive particles to coexist with the fine metal particles and the aluminum oxide to form conduction paths. It is therefore difficult to bring all deposited particles into contact with the conductive particles, and that part of the deposited particles which remain electrically isolated from the conduction paths has contributed to electrical resistance. Because of this, it has been impossible to attain the high activity necessary for realizing a solid oxide electrochemical cell.